Once you start using rsnapshot and you haven’t thought about increasing hard drive requirements, you didn’t setup LVM, you might want to migrate yours rsnasphot directory to bigger drive, preferably under LVM. You might see how to setup LVM here When it comes to migrate your current rsnapshot backup to other drive, you have two ...

To get UUID of the partition /dev/sdc3, run: blkid /dev/sdxx You might use it to create /etc/fstab entry for this drive, if you can expect that it might get different letters of drive (for example it is USB drive)

Want to be sure new drive is fine (has no badblocks?) Run badblocks -wvs /dev/sdx (might take ~50 hours on 3TB HDD). It will write down specific patterns on HDD, verifying them (four different patterns for whole drive). It will wipe out your data on sdX